In order to diversify functionality of a programmable logic device and promote implementation thereof on electronic equipment and the like, reduction in size of a switch connecting logic cells with one another, and reduction in ON-resistance of the switch are required. A switching element utilizing precipitation of metal in an ion conduction layer in which a metal ion conducts, is known to have a smaller size and less ON-resistance than a conventional semiconductor switch.
The switching element includes a two-terminal switch disclosed in Patent Literature 1 and a three-terminal switch disclosed in Patent Literature 2. The two-terminal switch has a configuration in which an ion conduction layer is sandwiched between a first electrode supplying a metal ion and a second electrode not supplying an ion. Switching is performed between the two electrodes by formation and disappearance of a metal bridge in the ion conduction layer. The two-terminal switch has a simple configuration, and therefore a manufacturing process thereof is simple, and an element size can be reduced to the order of nanometers. The three-terminal switch has a configuration in which second electrodes of two two-terminal switches are integrated, and secures high reliability. It is preferable to use a porous polymer having silicon, oxygen, and carbon as main components, as the aforementioned ion conduction layer. A porous polymer ion conduction layer is able to maintain high dielectric breakdown voltage even when a metal bridge is formed, and therefore has excellent operational reliability (Patent Literature 3). In order to integrate the switching element as a wiring selector switch in a programmable logic device, densification by size reduction of the switch and simplification of a manufacturing process are required. A wiring material of a leading-edge semiconductor device is mainly composed of copper, and therefore a technique of efficiently forming a variable resistance element in a copper wiring is expected. A technology of integrating a switch element utilizing an electrochemical reaction into a semiconductor device for a two-terminal switch is disclosed in Patent Literature 4, and the technology for a three-terminal switch is disclosed in Patent Literature 5. The literatures describe a technology of using a copper wiring on a semiconductor substrate also as a first electrode of a switch element. Use of the configuration enables reduction in step for newly forming a first electrode. Consequently, a mask for creating a first electrode becomes unnecessary, and the number of photomasks (PR number: number of photoresist masks) to be added for manufacturing a variable resistance element can be two. At this time, when an ion conduction layer (second ion conduction layer) is directly formed on a copper wiring, a surface of the copper wiring is oxidized, causing increase in leak current, and therefore a metal thin film functioning as an oxidation sacrificial layer is arranged between the copper wiring and the ion conduction layer. The metal thin film is oxidized by oxygen contained in the ion conduction layer, and becomes part of the ion conduction layer (first ion conduction layer). A metal constituting the oxidation sacrificial layer forms an alloy layer at an interface with copper, and, when a metal bridge is formed by voltage application, the metal is taken into the metal bridge. Non Patent Literature 1 discloses that retentive strength (retention characteristic) is improved by enhanced thermal stability of a metal bridge by the metal diffused into the bridge. At that time, generation efficiency of Joule heat is enhanced by the metal being taken into the metal bridge, and therefore current required for transition from “ON” to “OFF” does not increase.